This invention relates generally to materials and more particularly to materials which are transparent to radio frequency energy.
As is known in the art, there is a need for durable and strong components comprised of materials which have a high degree of radio frequency transparency. One application for such materials is as a radome to isolate a radar system from an external environment. Such radomes are often used on flight vehicles such as active or semi-active guided missiles.
One material used in the art for providing radomes is so-called “slip cast fused silica”. Silica (silicon dioxide) is one of a limited class of materials which has a very high viscosity at its softening temperature. This characteristic permits silica to form as either a vitreous material (i.e. glass), that is a material having no apparent crystal structure commonly referred to as fused silica, or alternatively, as a devitrified material that is a material having a definite crystal structure (i.e. ceramic).
For radomes used on flight vehicles, vitreous or fused silica material is generally used. Fused silica, the name given to the vitreous form, is preferred over devitrified or crystalline silica because the fused silica has a relatively low and isotropic thermal expansion coefficient compared to devitrified or crystalline silica. That is, fused silica has a coefficient of thermal expansion which is substantially independent of temperature over a relatively wide range of operating temperatures. This property of fused silica permits radomes of fused silica to exhibit excellent thermal shock resistance (Ts). Thermal shock resistance is generally characterized by the equation Ts=σ·K/E·α, where σ is strength, K is the thermal conductivity of the material, E is Young's Modulus, and α is the coefficient of thermal expansion.
One technique used for making fused silica bodies is so-called slip casting. In the slip cast technique, an aqueous slurry of silica cullet is prepared and introduced into a mold comprised of a material such as plaster of paris having the desired shape and size. The plaster of paris mold has the capacity for withdrawing water from the aqueous slurry leaving behind a rigid cast of silica which forms on the inside of the plaster of paris mold. The rigid cast is removed from the mold and allowed to finish drying over a period of several weeks until the cast has a green or pre-fired density in the range of 85% to 90% of theoretical density. During this drying process, controlled humidity environments are often used to prevent cracks from forming and propagating in the green cast. The green cast is then fired or sintered to achieve a final density of about 89%-90% of theoretical density.
Thus, fused silica fabricated for radome applications has so-called open porosity or small micropores which are disposed throughout the material. These micropores inhibit the propagation of surface flaws and cracks through the material which otherwise could cause a catastrophic failure of the radome. Crack propagation is often initiated by water droplet impact or so-called rain erosion which occurs when the radome traverses a rain field at a high velocity. The pores in the fused silica serve to retard such crack propagation. A material such as silicone is impregnated into the pores to prevent water from entering the radome. Water absorption in fused silica will cause undesirable r.f. absorption bands in the material. Such absorption is undesirable for radome applications.
Slip casting is a relatively expensive and time consuming process having a relatively low yield because of the critical yet slow drying step required to achieve the high green state density. Moreover, the sintering employed to achieve final density is also not particularly effective in strengthening the slip cast fused silica because the sintering must occur in this process over relatively short time and low temperature conditions. Higher time and temperature conditions are necessary to achieve additional strengthening of the slip-cast sintered, fused silica. However, higher time and temperature sintering conditions will close-off pores in the fused silica and will also cause the silica to crystallize and form “crystobalite”. Formation of a significant crystobalite phase in the fused silica is particularly undesirable, since the crystobalite provides the radome with an anisotropic component of thermal expansion which reduces the overall thermal shock resistance of the radome.
For a given density, conventional slip cast fused silica has a relatively low flexural strength characteristic. Typically, the flexural strength of slip cast fused silica having a density of 1.95 g/cm3 (approximately 88.6% of theoretical density) is about 6600 psi.
The microstructure of conventional slip cast fused silica is generally as that shown in FIG. 3A. This microstructure exhibits a preponderance of particles having an average size of 30 to 50 microns. Moreover, the microstructure is in general coarse and irregular in morphology which leads to the relatively low strength characteristic mentioned above.